The degradation of
intermediate temperature solid oxide fuel cell (ITSOFC) cathodes has
been identified as a major issue limiting the development of ITSOFCs
as high efficiency energy conversion devices. In this work, the effect
of Cr poisoning on (La<sub>0.6</sub>Sr<sub>0.4</sub>)<sub>0.95</sub>(Co<sub>0.2</sub>Fe<sub>0.8</sub>)­O<sub>3‑δ</sub> (LSCF6428),
a particularly promising ITSOFC cathode material, was investigated
on symmetrical cells using electrochemical impedance spectroscopy
and multiscale structural/chemical analysis by advanced electron and
ion microscopy. The systematic combination of bulk and high-resolution
analysis on the same cells allows, for the first time, direct correlation
of Cr induced performance degradation with subtle and localized structural/chemical
changes of the cathode down to the atomic scale. Up to 2 orders of
magnitude reduction in conductivity, oxygen surface exchange rate,
and diffusivity were observed in Cr poisoned LSCF6428 samples. These
effects are associated with the formation of nanometer size SrCrO<sub>4</sub>; grain boundary segregation of Cr; enhanced B-site element
exsolution (both Fe and Co); and reduction in the Fe valence, the
latter two being related to Cr substitution in LSCF. The finding that
significant degradation of the cathode happens before obvious microscale
change points to new critical SOFC degradation mechanisms effective
at the nanometer scale and below.